It is known in the art relating to the manufacture of crankshafts to use a grinding wheel to finish grind the crank pins which are eccentrically mounted with respect to the crankshaft axis of rotation. Typically the pins are first rough machined either in milling or turn-broaching operation, and then a pin grinder with a bonded grinding wheel is used to finish grind the pin. The grinding machine dresses the grinding wheel and then the pins are finish ground one pin at a time, thereby resulting in long cycle times. To carry out the grinding step, the crankshaft is rotated about the axis of the pin being ground and must be indexed to another axis of rotation for each differently angled pin. This indexing step further contributes to the long cycle time.
A previously proposed crankshaft grinder allows the crankshaft to be rotated about the axis of its main journals and a grinding wheel is synchronously moved in and out as the crankshaft turns to grind the pin in the correct cylindrical shape. Because of the bulky nature of the grinding machine only one pin at a time can be ground.
It is known in the related art of grinding the cams of camshafts to use a belt grinder. Because several belts can be operated side-by-side it is practical to grind several cams simultaneously. An example of such a cam grinder is shown in U.S. Pat. No. 4,833,834 to Patterson et al, entitled "Camshaft Belt Grinder" which is assigned to the assignee of the present invention, and which is incorporated herein by reference. There the camshaft is rotated about its axis and for each cam, a grinding belt is urged against the cam by a grinding head which is moved in and out according to the desired cam shape. The head motion is coordinated with the camshaft rotation and is driven by a mechanical cam arrangement or by a servo mechanism controlled by a computerized numerical control (CNC) unit. Each belt is driven by a drive wheel and its path is around the head, a pair of stationary guide wheels on either side of the head, and a tension wheel which is biased to maintain a desired tension on the belt and moves transversely to the belt path to compensate for the movement of the head. Since the cam is relatively small, only small movements of the grinding head and of the tension wheel take place.
It is proposed to use a belt grinder similar to the camshaft grinder to grind crankshaft pins. While such an application of a belt grinder may seem to be straightforward, there is a significant difference between the camshaft and the crankshaft application: while the cam is relatively small and requires a small motion of the grinding head, say 3/8 inch, the throw of a crankshaft pin is large and requires a large motion on the order of four inches of the grinding head and a corresponding large motion of the tension wheel if the same apparatus is used. The result is excessive travel of the tension wheel which causes erratic motion of the tension wheel and changes in belt tension during a grind cycle. In addition, the angle of the belt to the head and the tension determine the pressure of the belt against the head. When the head travel is great the angle changes significantly so that the belt pressure is also erratic.